I have an above average sized head, and the large size is a must if you're looking for a comfortable mask.
I had reservations about mask wearing. I was concerned about the amount of polluting particles entering my lungs during long commutes through central London, and after a few too many bad coughs, decided to go for the Respro Sportsta.
After two months of wearing it daily, I'm very satisfied I became a mask wearer. It is comfortable, it doesn't steam up my glasses and it keeps a lot of bad stuff out of my lungs. I have to change the filter monthly, which is easy to do, and it takes no time at all to put on - so is worthwhile regardless of the length of your journey, but especially worthwhile if you're cycling for an hour as I do.
Breathing through the mask is no problem at all. In winter it is especially helpful as the breaths you take in tend to be nice and warm, rather than the icy blasts you get if you're maskless.
The negatives are minor and come with the concept of a mask rather than this particular brand. The mask is a touch tight on the nose, which is uncomfortable and requires the odd quick adjustment when stopped at lights. The moisture also gets the mask a little wet inside. This isn't to the detriment of comfort, but does make the mask look a little disgusting when you take it off.
All in all though, if you're considering a mask to cycle in, this is a fine, affordable choice.